This invention relates to electric lamps primarily designed for illuminating rooms and having a light diffuser disc which is provided, at least on one face, with a prism structure for deflecting light and for distributing the light density. The light source, such as an electric bulb (point-like light source) or a fluorescent tube (linear light source) is situated at a distance from the diffuser disc.
For designing the illumination in offices, work halls, classrooms and the like, a number of criteria has to be taken into consideration to achieve a "correct" and economical lighting. Thus, the lamps have to be arranged in such a manner that there is achieved, in the entire room, a substantially uniform light intensity distribution and further, for each work position, there is obtained an approximately uniform light density (brightness). Further, the possibility of glare, either by reflection from a surface, such as a desk, or by direct radiation from a lamp situated in the line of vision of a person in his working position, should be excluded.
In earlier attempts these multiple, often partially contradictory criteria were sought to be met by providing the lamp with a closure of translucent or matte glass or translucent synthetic glass. Recently, because of the poor light efficiency resulting from the use of translucent glass, transparent material has been used for lamp closures in order to increase the light efficiency. In order to meet the problems of light density (brightness) and light distribution which, in case of transparent material, is substantially more difficult, at least one face of the transparent diffuser disc has been provided with a prism-like structure with which a light distribution of a certain degree could be effected by means of light deflection. While, with the known diffuser discs having such a prism structure, the problem of the so-called direct glare could be substantially solved, problems of the glare by reflection, that is, the limitation of the admissible maximum value of the light density in an angular range of 0.degree.-45 .degree. (that is, immediately below the lamp) could, however, not be satisfactorily solved for the required light intensities. The light distribution curves, which can be obtained with prism structures known heretofore, require the use of a relatively large number of lamps to achieve a substantially uniform light distribution illumination in the entire room for avoiding "holes" in the room lighting.